1. Field of the Invention
This invention relates to a method of forming, by the flame hydrolysis technique, high optical purity blanks from which high quality optical waveguides, lenses, prisms and the like can be made. This invention is particularly applicable to optical waveguides which must be formed from extremely pure materials.
High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in communication traffic. These systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical fiber having a transparent core surrounded by transparent cladding material having a refractive index lower than that of the core.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in fiber optic form. Glass preparation techniques which have shown much promise are based on the flame hydrolysis process which employs vapor phase reaction of high purity vapors. This approach to the formation of low loss optical waveguides is based on methods described in U.S. Pat. Nos. 2,272,342 and 2,326,059 issued to J. F. Hyde and M. E. Nordberg, respectively. The flame hydrolysis technique has been employed to prepare single mode waveguides and multimode waveguides of both the step-index and graded-index type. Various methods employing the flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Pat. Nos. 3,711,262; 3,737,292 and 3,737,293. A method employing the flame hydrolysis technique to form a graded-index type waveguide is taught in U.S. patent application Ser. No. 239,496 filed Mar. 30, 1972, entitled "Method of Forming a Light Focusing Fiber Waveguide" now U.S. Pat. No. 3,826,560.
The usefulness of glass optical waveguides in optical transmission systems depends upon the attainment of very low loss transmission over the entire wavelength range of about 700-1100 nm. This can be achieved by reducing attenuation due to optical scattering and absorption to a level which approaches the minimum theoretically attainable level. Waveguides in which at least 80% of the scattering loss can be accounted for by intrinsic glass scattering have been made by the aforementioned flame hydrolysis technique. However, due to the presence of residual water produced by this technique, absorption losses between 700 nm and 1100 nm have been excessively large. By residual water in glass is meant that the glass contains a high level of OH, H.sub.2 and H.sub.2 O. One explanation of residual water may be found in U.S. Pat. No. 3,531,271 to W. H. Dumbaugh, Jr. The maximum attenuation in the aforementioned wavelength range that is attributable to residual water occurs at about 950 nm and is related to the OH content by the coefficient 1.25 dB/km/ppm OH. The remaining portion of the attenuation at 950 nm, which is due to factors such as intrinsic material scattering, amounts to about 4 dB/km. For example, a glass optical waveguide having an attenuation less than 6 dB/km at 800 nm may have an attenuation greater than 100 dB/km at 950 nm due to the presence of water therein. To be useful in optical communication systems, optical waveguide attenuation is preferably less than 10 dB/km at the wavelength of light being propagated therein. In order to achieve such low attenuation over the entire range between 700 nm and 1100 nm, a glass waveguide fiber must be rendered substantially water-free, i.e., the amount of residual water within the fiber must be reduced to a level of less than 10 ppm.
Since residual water causes a strong absorption at about 2.73 .mu.m, but light transmission at about 2.1 .mu.m is relatively unaffected by water, residual water content in a glass waveguide may be specified in terms of an absorption coefficient referred to as the "beta value" and designated .beta..sub.OH, which is calculated from the formula: ##EQU1## wherein t is the glass thickness in mm, T.sub.2.1 and T.sub.2.73 are the transmittances in percent at 2.1 .mu.m and 2.73 .mu.m, respectively, .beta..sub.OH being in terms of mm.sup..sup.-1. To produce waveguides having an attenuation less than 20 dB/km over the range 700-1100 nm, it has been found that the waveguide glass must have a .beta..sub.OH of less than 0.001.
2. Description of the Prior Art
Since it is impossible to reduce the water content to acceptable levels after flame hydrolysis-produced soot has been consolidated to form a solid glass coating, the water must be removed before or during the consolidation process. Heretofore, various methods were employed to reduce the water content in optical waveguides produced by flame hydrolysis. Such disadvantages as long processing times, equipment problems and incomplete water removal were encountered.
One prior art method of producing low water content fused silica included the steps of forming by flame hydrolysis a SiO.sub.2 soot preform and then placing the preform in a preheated furnace at approximately 1500.degree.C. for approximately 30 minutes. The furnace contained a reducing atmosphere of cracked ammonia or forming gas. During the heat treatment, the soot was sintered and consolidated into a dense glass body which was to a certain extent water-free (.beta..sub.OH approximately equal to 0.02), but the amount of water remaining in the resultant glass was excessive in terms of tolerable amounts for optical communication systems.
A .beta..sub.OH value of about 0.01 was achieved by consolidating a soot preform in an inert dry atmosphere such as nitrogen, helium, neon or argon. In accordance with this method, which is disclosed in copending patent application Ser. No. 239,742 filed Mar. 30, 1972, the inert gas replaces trapped air in the preform and subsequently dissolves in the glass. Since this method includes gradient sintering, gases can escape through unconsolidated parts of the preform. Optical waveguides made by the process exhibited attenuations as low as 30 dB/km at 950 nm, a value that is not sufficiently low for the propagation of optical signals.
In my copending patent application Ser. No. 239,746 filed Mar. 30, 1972, there is disclosed a method of forming a glass optical waveguide containing less than 20 ppm residual water. In accordance with the method of that application the flame hydrolysis-produced soot preform is placed in a chamber which is evacuated to less than 10.sup..sup.-5 Torr. The chamber is heated below the sintering temperature of the soot to permit entrapped gas to escape from the preform and the temperature is maintained until an equilibrium is reached between the partial pressure of the entrapped gas in the porous preform and the partial pressure of the same gas in the furnace environment. A period of about 24 hours is required for equilibrium to be reached, at which time the porous preform is further heated to consolidate the soot particles and to form a dense glass member. An optical waveguide formed in accordance with this method exhibited an attenuation of less than 20 dB/km. Although relatively low loss optical waveguides can be produced, this process is disadvantageous in that it requires an extremely long time for water removal, and it may result in the volatilization of some dopant oxides. Also, equipment problems have been encountered because of the need to maintain very low pressures for long periods of time. Moreover, the preform cannot be consolidated until after the water removal step is completed.
Various methods have been heretofore employed to make low water content glasses by methods other than the flame hydrolysis technique. None of these methods have been found to satisfactorily remove water from a flame hydrolysis-produced soot preform which is to be used in the manufacture of optical waveguides. For example, the aforementioned patent to W. H. Dumbaugh, Jr. teaches a method of making a low water content glass body by mixing the batch ingredients together with an effective amount of a chemically-reactive, chlorine containing agent and melting the glass in the presence of a dry atmosphere flowing directly over the glass melt. Obviously, this method cannot be adapted to the flame hydrolysis process wherein the glass article is not formed by melting batch ingredients. Moreover, it is noted that even though chlorine containing compounds such as SiCl.sub.4 are employed in the flame hydrolysis process to form silica containing soot preforms, the chlorine present does not result in the formation of water-free soot.
Another prior art method for removing water from glass bodies produced by a technique other than flame hydrolysis is disclosed in U.S. Pat. No. 3,459,522 issued to T. H. Elmer et al. This patent describes a method of removing residual water from a porous, high silica content glass body by subjecting it to a flowing stream of a substantially dry atmosphere containing 10% or more of either chlorine gas or a chlorine vapor at a temperature of 600.degree.-1000.degree.C. The treated porous glass body is thereafter consolidated in a dry, nonoxidizing atmosphere to produce a nonporous, transparent glass article. The porous glass body disclosed in the Elmer et al. patent is well known under the commercial designation "96% silica glass", which is produced by consolidating a porous glass body characterized by a multiplicity of intercommunicating, submicroscopic pores throughout its mass. The basic production steps involved in the formation of such a porous body, which are described in U.S. Pat. No. 2,221,709 issued to H. P. Hood et al., include the steps of forming an article from a borosilicate glass, thermally treating the article at a temperature of 500.degree.-600.degree.C. to separate the glass into a silica-rich phase and a silica-poor phase, leeching the silica-poor phase to produce a porous structure composed of the silica-rich phase, removing the leeching residue, and thermally consolidating the porous structure into a nonporous vitreous article.
Because of the kind of microstructure present in the porous glass body disclosed in the Elmer patent and due to the fact that the chlorination process disclosed therein is carried out at a temperature below the consolidation temperature, an atmosphere containing a relatively large concentration of chlorine must be employed. That patent therefore requires a chlorine containing atmosphere having 10% or more of either chlorine gas or a chlorine vapor, and most of the examples disclosed therein employ chlorine gas with no diluent. The Elmer patent further teaches that after chlorine treatment, it is undesirable to maintain the porous glass in a chlorine containing atmosphere while the temperature is increased to the consolidation temperature because of economic considerations and because this may result in retention of an excess amount of chlorine within the glass and may cause splitting of the glass. Therefore, the chlorine treated porous glass is removed from the chlorine atmosphere and transferred in an inert atmosphere such as nitrogen for further heat treatment, consolidation being preferably performed in an inert atmosphere or vacuum at a temperature between 1200.degree.C. and 1300.degree.C.
For at least the following reasons the method of the Elmer et al. patent is unsatisfactory for removing water from flame hydrolysis-produced glass soot preforms from which optical waveguides are made. The high chlorine content of the chlorine containing atmosphere employed by the Elmer et al. patent can cause reboil in subsequent heat treatment of the preform and can also introduce an unacceptable level of contamination in the glass due to the presence of contaminants in commercial grade chlorine sources. Transferring the soot preform from the chlorination chamber to the consolidation chamber can permit water to reenter the porous soot blank. Whereas the Elmer patent teaches separate chlorination and consolidation steps, it is more economical and efficient to remove water from the glass soot concurrently with the consolidation of such soot. Moreover, the rate of removal of water by chlorine is temperature related, it being slower at temperatures between 600.degree.C. and 1000.degree.C. than at the soot consolidation temperature which is between about 1250.degree.C. and 1700.degree.C. for silica.